1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit having a lamp of a bent shape, to enhance the luminance uniformity in a backlight unit having a division structure.
2. Discussion of the Related Art
A cathode ray tube (CRT), one of flat display devices, has been widely used for monitors of a television, a measuring machine and an information terminal. However, the CRT cannot satisfy demands for a compact size and lightness in weight. Thus, display devices such as a liquid crystal display (LCD) device based on optoelectronic principles, a plasma display panel (PDP) using a gas discharge and an electroluminescence display (ELD) device based on an electric field luminous effect have been studied to substitute for the CRT.
Among those display devices, the LCD device has been most actively studied. The LCD device has the advantageous characteristics such as compact size, lightness and low power consumption, and is suitable for ultra-thin flat display devices, for example, monitors for spacecrafts, aircrafts, notebook computers, laptop computers, desktop computers and large-sized display devices. This explains the strong demand for the LCD devices today.
Most LCD devices control the light transmittance from ambient light to display images. To enhance the luminance, it is necessary to form an additional light source such as a backlight unit in an LCD panel. In general, the backlight unit used as the light source of the LCD device has a cylindrical fluorescent lamp, which is classified into two types, a direct type and an edge type, according to the arrangement of the fluorescent lamps.
For the edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit includes a lamp for emitting light, a lamp holder inserted into both ends of the lamp to protect the lamp, and a reflective sheet having one side inserted to the light-guiding plate and surrounding the circumference of the lamp, to reflect the light emitted from the lamp to the light-guiding plate. In this respect, the edge type backlight unit is generally applied to relatively small sized LCD devices such as the monitors of the laptop computer and the desktop computer, which is advantageous in that it has the features of great uniformity of light, long life span, and thin profile.
With trend of the large-sized LCD device of 20-inch or more, the direct type backlight unit is actively developed, in which a plurality of lamps are formed along one line on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with the light. The direct type backlight unit, which has greater light efficiency as compared with the edge type backlight unit, is used for the large-sized LCD device requiring high luminance. That is, the LCD device of the direct type backlight unit is generally used for the large-sized monitor or television. The large-sized monitor or television may be driven for a long time, and has a plurality of lamps. Accordingly, it is probable that some of the lamps may be out of order and do not emit light.
In case of the edge type backlight unit, even though one lamp fails, it only causes a slight decrease in luminance and will not significantly affect the luminance on the entire LCD panel. However, in case of the direct type LCD device, a plurality of lamps are provided underneath a screen of the LCD panel. Accordingly, if one of the lamps fails due to some reasons such as reaching the lifespan of lamp, a screen portion corresponding to the failed lamp becomes darker as compared with surrounding portions of the screen. Accordingly, it is necessary for the direct type LCD device to have a simple structure suitable for disassembly and assembly of the lamp unit.
Hereinafter, a related art backlight unit will be described with reference to the accompanying drawings. First, an edge type backlight unit according to the related art will be described as follows. FIG. 1 is an exemplary view of an edge type backlight unit according to the related art. As shown in FIG. 1, the edge type backlight unit according to the related art includes an LCD panel 14, a lamp 10, a light-guiding plate 11, a light-diffusion sheet 12, a prism sheet 13, a fixation structure 15, and a lower reflective sheet 16. The LCD panel 14 displays images thereon by controlling the light transmittance, and the lamp 10 is used as a light source. Also, the light-guiding plate 11 guides the incident light, emitted from the lamp 10, to the LCD panel 14. Then, the light-diffusion sheet 12 diffuses the light guided by the light-guiding plate 11 to a predetermined angle. The prism sheet 13 concentrates the diffused light, and sends the concentrated light to the rear of the LCD panel 14. Also, the fixation structure 15 is positioned below the light-guiding plate 11. To minimize the loss of light, the lower reflective sheet 16 reflects the light progressing toward the fixation structure 15 back to the LCD panel 14.
In addition, the edge type backlight unit according to the related art further includes lamp reflective sheets 18 and lamp holders 17. Each of the lamp reflective sheets 18 covers the lamp 10 except for a light-incidence surface of the light-guiding plate 11, thereby decreasing the loss of light incident to the light-incidence surface of the light-guiding plate 11 from the lamp 10. Also, the lamp holders 17 are positioned at both ends of the lamp 10, to fix the lamp 10 to a predetermined position, and to maintain a predetermined interval between the light-incidence surface of the light-guiding plate 11 and the lamp 10.
In the aforementioned edge type backlight unit, a plurality of light-diffusion sheets 12 and prism sheets 13 may be deposited. If the edge type backlight unit according to the related art is used for the monitor, each lamp 10 is positioned on both sides of the light-guiding plate 11. However, in case the lamp edge type backlight unit is used for the notebook computer, the lamp may be positioned on one side of the light-guiding plate 11.
In the meantime, a direct type backlight unit according to the related art will be described as follows. FIG. 2 is an exemplary view of a direct type backlight unit according to the related art. Referring to FIG. 2, the direct type backlight unit according to the related art includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c. The fluorescent lamps 1 are arranged at fixed intervals along one direction. The outer case 3 fixes and supports the fluorescent lamps 1. Then, the light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown).
The light-scattering means 5a, 5b and 5c are used to prevent the silhouette of the fluorescent lamps 1 from being reflected on the display surface of the LCD panel (not shown), and provide a light source with uniform luminance. To improve the light-scattering effect, there are a plurality of diffusion sheets and one diffusion plate between the fluorescent lamps 1 and the LCD panel. Also, a reflective sheet 7 is provided on an inner surface of the outer case 3 for reflecting the light emitted from the fluorescent lamps 1 to the display part of the LCD panel, thereby improving light efficiency.
At this time, the fluorescent lamps 1 are formed of Cold Cathode Fluorescent Lamps CCFL. Specifically, electrodes are formed at both ends of a tube in each of the fluorescent lamps 1. Accordingly, when a voltage is applied to the electrodes of the fluorescent lamps 1, the fluorescent lamps 1 emit the light. Also, the both ends of the fluorescent lamp 1 are respectively inserted to holes formed on both sides of the outer case 3.
Then, the both electrodes of the fluorescent lamp 1 are connected with power supplying wires 9 and 9a so that the power for operating the fluorescent lamp 1 can be supplied to the fluorescent lamp 1. The power supplying wires 9 and 9a are connected to a driving circuit by an additional connector. Thus, each fluorescent lamp 1 requires the individual connector. That is, the power supplying wire 9 connected to one electrode of the fluorescent lamp and the power supplying wire 9a connected to the other electrode of the fluorescent lamp are connected to a single connector. Also, any one of the power supplying wires 9 and 9a is curved to the lower side of the outer case 3 for being connected with the connector.
FIG. 3 is an exemplary view of another direct type backlight unit according to the related art. As shown in FIG. 3, the direct type backlight unit uses a plurality of U-shaped fluorescent lamps 31. In each of the U-shaped fluorescent lamps 31, electrodes are formed at the both ends, wherein the electrodes are connected to respective connectors 33a and 33b of an inverter 32. In FIG. 3, an outer case 30 is provided for supporting the fluorescent lamps 31.
However, both the edge type backlight unit and the direct type backlight unit according to the related art have several disadvantages.
First, although the edge type backlight unit according to the related art can improve the uniformity of light with the light-scattering means 5a, 5b and 5c, it is impossible to control the luminance of displayed image in a particular region since it uses the light-guiding plate 11.
Although there are some methods for improving the picture quality, for example, a method of sequentially turning on the plurality of U-shaped fluorescent lamps 31, it still has the limitation controlling the luminance of a particular region of the screen. Furthermore, if the U-shaped fluorescent lamps 31 are partially overlapped to one another, the luminance may be lower in some particular regions.